Segmental specialization of calcium-activated potassium conductances in an identified leech neuron.

1. Retzius (R) neurons of the fifth and sixth segmental ganglia of the leech, called R(5,6) neurons are specialized to innervate the adjacent reproductive organs and are morphologically and functionally distinct from R neurons of standard ganglia [R(x) cells]. In this study the electrical properties of the R(x) and R(5,6) neurons were compared under current-clamp and voltage-clamp conditions. 2. The action-potential waveforms of R(x) and R(5,6) cells were similar except for the presence in the R(5,6) cells of a long afterhyperpolarization (AHP) following action potentials arising from the resting membrane potential but not from more depolarized potentials. Its role may thus be to inhibit firing of the R(5,6) neurons at rest or in response to weak depolarizing stimuli. 3. In the presence of the Ca2+ channel blocker Cd2+, the long AHP of the R(5,6) was abolished, and the action potentials of all R cells were identical. 4. Under voltage clamp, current kinetics and densities were similar between R(x) and R(5,6) cells for Ca2+ currents, delayed and inward rectifier K+ currents, and a rapid Ca(2+)-activated K+ current (IKc) that is common to the two cell types. The R(5,6) cells, however, expressed a second Ca(2+)-activated K+ current that was not observed in the R(x) cells. This current, called IKAHP, activated and inactivated more slowly than IKC, with a time course similar to that of the AHP observed under physiological conditions. 5. Neither IKC nor IKAHP was blocked by high concentrations of charybdotoxin or apamin, which block vertebrate Ca(2+)-activated K+ channels.(ABSTRACT TRUNCATED AT 250 WORDS)